The present invention generally relates to an implantable atrial defibrillator and method for applying cardioverting electrical energy to the atria of a patient's heart in need of cardioversion. The present invention is more particularly directed to such a defibrillator which includes an atrial fibrillation detector for determining if the atria are in need of cardioversion and a therapy sequence control which terminates cardioversion therapy when the atrial fibrillation detector fails at least twice in succession to determine a need for cardioversion of the atria of the heart.
Atrial fibrillation is probably the most common cardiac arrhythmia. Although it is not usually a life threatening arrhythmia, it is associated with strokes thought to be caused by blood clots forming in areas of stagnant blood flow as a result of prolonged atrial fibrillation. In addition, patients afflicted with atrial fibrillation generally experience palpitations of the heart and may even experience dizziness.
Atrial fibrillation occurs suddenly and many times can only be corrected by a discharge of electrical energy to the heart through the skin of the patient by way of an external defibrillator of the type well known in the art. This treatment is commonly referred to as synchronized cardioversion and, as its name implies, involves applying electrical defibrillating energy to the heart in synchronism with a detected ventricular electrical activation (R wave) of the heart. The treatment is very painful and, unfortunately, most often only results in temporary relief for patients, lasting but a few weeks.
Drugs are available for reducing the incidence of atrial fibrillation. However, these drugs have many side effects and many patients are resistant to them which greatly reduces their therapeutic effect.
Implantable atrial defibrillators have been proposed to provide patients suffering from occurrences of atrial fibrillation with relief. Unfortunately, to the detriment of such patients, none of these atrial defibrillators has become a commercial reality.
Two such proposed defibrillators, although represented as being implantable, were not fully automatic, requiring human interaction for cardioverting or defibrillating the heart. Both of these proposed defibrillators required the patient to recognize the symptoms of atrial fibrillation, with one defibrillator requiring a visit to a physician to activate the defibrillator, and the other defibrillator requiring the patient to activate the defibrillator with an external magnet. Neither defibrillator included an atrial fibrillation detector or detected atrial activity of the heart. As a result, these manually operated defibrillators provided no atrial fibrillation detection support for the patient.
An improved implantable atrial defibrillator which is automatic in operation is fully described in U.S. Pat. No. 5,282,837. The atrial defibrillator disclosed in this patent is truly automatic by including an atrial fibrillation detector which, responsive to sensed atrial activity, determines when the atria of the heart are in need of cardioversion. When the atrial fibrillation detector determines that the atria are in fibrillation and thus in need of cardioversion, the atrial fibrillation detector causes a cardiovertor stage to deliver defibrillating or cardioverting electrical energy to the atria in time relation to a detected ventricular electrical activation (R wave) of the heart. As a result, the atria are automatically and safely cardioverted.
Atrial fibrillation, unlike ventricular fibrillation, is generally not immediately life threatening. As a result, once atrial fibrillation is detected, a therapy sequence may be initiated which provides for delivery of a number of cardioverting shocks having peak voltages which incrementally increase from a relatively low level to eventually a relatively high level. Therapy may then be terminated as soon as the atrial fibrillation detector confirms successful cardioversion. Such a therapy sequence preferably includes detection for atrial fibrillation prior to each shock to be delivered and after each shock is delivered. This sequence may continue until a predetermined number of cardioversion attempts have been made.
While it is, of course, beneficial to the patient for the atrial fibrillation detector to detect for atrial fibrillation prior to and after each attempted cardioversion, such repeated detection can cause a problem of its own. No fibrillation detector is one hundred percent accurate (one hundred percent sensitivity with one hundred percent specificity) in determining if there is a need for cardioversion. Hence, if numerous detections are required in a therapy sequence, due to numerical probabilities, there will be some finite possibility that the detector will fail to detect the need for further cardioversion and terminate the therapy when there actually exists a need for further cardioversion. The end result of this would be that the fibrillation would not be successfully cardioverted and would persist.
The present invention provides a simple, yet elegant solution to this problem. To accomplish this end, the present invention provides an atrial defibrillator and method wherein a therapy sequence is initiated upon a first detection of atrial fibrillation with a first sensitivity, and wherein the therapy sequence is terminated when the atrial fibrillation detector, with a second and greater sensitivity, fails to detect a need for further cardioversion. The sensitivity of the detector is thus increased after therapy is initiated to significantly reduce the probability that the presence of atrial fibrillation will go undetected during a therapy sequence.